1. Field of the Invention
The present invention relates to a light-weight advanced grid structure having low thermal expansion characteristics, which is obtained by using carbon fiber reinforced plastic serving as an aerospace material which is lighter than metal and has a low thermal expansion coefficient, and to a method of manufacturing the advanced grid structure and a space telescope using the advanced grid structure.
2. Description of the Related Art
In recent years, in various fields, there has been an increasing demand for high-resolution satellite images of objects in an aerospace and on the earth. Along with this, there has been a demand for development of an observation satellite having mounted thereon a telescope having a resolution higher than that of a conventional telescope.
In order to enhance the resolution of a telescope in such a satellite, it is necessary not only to enlarge a mirror but also to provide a light-weight mirror barrel structure having a low thermal expansivity for holding a large mirror thermally stably. This is because, when thermal dimensional stability is low in the case where a temperature distribution occurs in a telescope structure in a space environment, the structure is distorted, which results in a decrease in a resolution of a satellite image. Thus, it is important to realize a mirror barrel structure having a low thermal expansivity.
Further, the weight of a telescope, that is, the weight of a satellite increases along with the enlargement of a mirror. However, there is a limit to a load capacity because the satellite is set off with a rocket. Thus, it is also necessary to reduce weight of the mirror barrel structure.
As a mirror barrel structure that satisfies both the low thermal expansivity and the light weight, a cylindrical advanced grid structure has been proposed. In the cylindrical advanced grid structure, carbon fibers are oriented in a lattice shape in a cylindrical plane in a direction parallel to a cylinder axis direction and in a direction forming an angle of ±60° with respect to the cylinder axis direction. Such a cylindrical advanced grid structure has a low thermal expansivity in the cylinder axis direction and is light weight. Further, as a method of manufacturing such an advanced grid structure, there is generally known a method of forming an advanced grid structure by placing cores made of rubber or metal for forming a lattice on a forming die and laminating tape-shaped prepregs between the cores.
In the case where the core is made of rubber, a prepreg having a width larger than a desired rib width is laminated, and a forming pressure is applied to the prepreg by thermal expansion of the rubber and expansion of the rubber in an in-plane direction of the rubber, which is caused by the pressure from an out-of-plane direction, to thereby form the advanced grid structure into a desired dimension (for example, see S. M. Huybrechts and three others, “Manufacturing theory for advanced grid stiffened structure”, Composites: Part A33 (2002) 155-161).
On the other hand, in the case where the core is made of metal, a gap of the same dimension as a desired rib width is formed with use of the core, and the gap is filled with fibers and a resin so that a desired dimension is obtained (for example, see Japanese Examined Patent Publication No. Hei 4-41889).
However, the prior art has the following problems.
In S. M. Huybrechts and three others, “Manufacturing theory for advanced grid stiffened structure”, Composites: Part A33 (2002) 155-161, silicon rubber is used as a core made of rubber. The linear expansion coefficient of silicon rubber is very large (i.e., 200 ppm/K) and silicon rubber expands more greatly by heating during formation, compared with metal. Further, silicon rubber has a lower coefficient of elasticity compared with metal, and hence, even when the same forming pressure is applied to silicon rubber in an out-of-plane direction, a displacement amount of silicon rubber is larger than that of metal, and a displacement amount of silicon rubber in an in-plane direction ascribed to a Poisson ratio is also larger than that of metal.
The forming pressure in the in-plane direction is applied to the prepreg due to the above-mentioned thermal and mechanical expansion in the in-plane direction, and the dimension of the rib width is determined by the expansion amount of silicon rubber. The expansion amount of silicon rubber is determined by a temperature difference from room temperature and the forming pressure. Therefore, in order to obtain a desired forming accuracy, it is necessary to maintain a uniform temperature and forming pressure to be applied in all the cores.
However, in general, it is very difficult to maintain a uniform forming condition in all the cores, and the expansion amount varies depending also upon the dimensional accuracy of the core. Therefore, it is very difficult to obtain a desired forming accuracy through use of the core made of rubber.
On the other hand, in Japanese Examined Patent Publication No. Hei 4-41889, the problem of forming accuracy is solved by using a core made of metal. The thermal and mechanical deformation of metal is small, and hence, the dimension of the core does not change significantly during a formation process. Therefore, a gap formed by the forming die and the core is filled with fibers and a resin so that a desired dimension is obtained.
Herein, the advanced grid structure has a region in which ribs oriented in different directions cross each other. Then, in the case where fibers are laminated continuously with respect to one rib, in the region in which the ribs cross each other, in a lamination direction (thickness direction), the amount of carbon fibers becomes twice as large as the amount in a region in which the ribs do not cross each other. Thus, unevenness of a rib thickness or bending of fibers is caused by the difference in the amount of fibers, which results in a breakage point of a formed article.
In order to solve the above-mentioned problem, in Japanese Examined Patent Publication No. Hei 4-41889, prepregs are laminated with the shape separated in a plurality of patterns so that the amount of carbon fibers in the thickness direction is maintained uniform. However, in Japanese Examined Patent Publication No. Hei 4-41889, the prepregs are laminated with the shape separated in a plurality of patterns as described above, and hence, the fibers are not continuous with respect to one rib. Therefore, the original mechanical and thermal characteristics of the grid structure may not be exhibited.
In the cylindrical advanced grid structure, the thermal expansion coefficient in the cylinder axis direction is optimized by the area of the crossing region and the curvature of the cylinder, and the weight is optimized by the number of laminated prepregs. In order to realize a cylindrical advanced grid structure obtained by optimum design, it is necessary to form the cylindrical advanced grid structure so as to have the dimension and amount of carbon fibers with good accuracy as designed.